One can imagine... vs One could imagine...

1) The event was only available on audio feed, so it was impossible to know how the president reacted. But one could imagine a sly grin on his face. If only it were so simple.
(https://www.huffingtonpost.in/entry/gas-tax-hike_n_6263744?ri18n=true)

Q1) In general, what's the difference between "One can imagine..." and "One could imagine..."?

Q2) The sentence (1) uses "could" instead of "can". Is it because the context is set in the past tense, and therefore "can imagine" changes to "could imagine"? Or is the "could" in the conditional/hypothetical mood in that sentence?